1. Field of the Invention
This invention relates to measuring and testing electrochemical processes; and it relates particularly to processes for the evaluation of protection of large metallic structures against corrosion attack.
2. Description of the Prior Art
Large metallic structures located in land or sea regions, such as pipelines, wells, structural supports and the like are subjected to corrosion attack. To minimize this corrosion attack, these structures may be covered with protective materials such as inert wrappings of fiber or cloth, and exterior impervious coatings of bitumen, cement or the like. Unavoidably, some voids remain within the protective coatings during their application, or are created during the installation of the structure within the earth region or thereafter by mechanical injury. The exposed surfaces of the structure are exposed to corrosion.
Many schemes have been envisioned for protecting these structures. For example, galvanic or electrochemical corrosion attack is very much more common than the usual chemical attack envisioned by the presence of a corrodant. In the electrochemical corrosion attack, the structure is at a different potential level than adjacent galvanic material so that a current flow between the two occurs through the earth region even though it is of substantially high specific resistivity. The presence of oxygen or other type of corrosive medium accelerates the effect of the galvanic corrosion attack. Obviously, corrodants also exist in the earth region about the structure.
The primary method of retarding electrolytic corrosion attack is through the use of cathodic protection. In cathodic protection, the structure receives a flow of current from an anodic (anode) electrode spaced remotely in the earth region. The anode may be a metal, such as magnesium, which has a lower half-cell potential than the structure in the earth region. Alternatively, the anode may be inert, such as a carbon mass, and a flow of cathodic current provided from a suitable d.c. power source. In either event, the flow of cathodic current from the metallic structure to the anode is assumed to be of proper magnitude when the structure (steel) is about 0.25 to 0.30 volts negative relative to the earth region immediately surrounding the structure. A more reliable criterion of proper cathodic protection is the current density of the structure in the amount of at least 3 milliamperes per square foot of the structure in contact with the earth region. Under these conditions, the structure may be directly exposed to the earth formation and all of its proportions are protected against corrosion attack.
Many methods and arrangements have been proposed for evaluating the cathodic protection of a metallic structure. Generally, these techniques measure the potential of the structure in the earth region relative to a closely adjacent reference electrode, such as the copper sulfate half-cell. Unfortunately, the non-isotropic characteristics of the earth region prevent a correlation of the potential measurement relative to the required current flow magnitude from the remote anode(s) for the proper cathodic protection of the entire structure. Repeatedly, prior art schemes concerned themselves primarily with corrections for the resistivity of the earth region in the potential measurement between the structure and the reference electrode. These measurement techniques have failed to provide uniformally acceptable results.
The concept of cathodic protection of an "anodic" structure is a very sound approach to corrosion attack abatement. However, the theory and practices of surveying and monitoring such installations have been irrelevant and invalid because of the assumption that a separate "reference" electrode is required to determine proper cathodic current magnitudes. Fundamental electrical analysis of the problem leads to a different concept.
Cathodic protection of any portion of the structure exists when the anodic reaction rate (dissolution of metal ions) has been suppressed to a magnitude consistent with the corrosion allowance/life time criterion but without an excess suppression causing an accelerated cathodic reaction rate adequate to delaminate protective coverings, etc., or embrittle the structure due to excessive hydrogen generation.
The large metallic structure is a low resistance "conductor" surrounded by a more resistive earth region. Therefore, the structure's "potential" is essentially identical in all portions and throughout its extent. This may be illustrated by analogy with a balloon: the internal pressure (potential) is equal at all parts; and the balloon's walls are distorted by variations in externally applied forces which at some points distort the wall outward, and at others, inward. The stresses in the balloon's wall vary considerably in amount and place depending on variation in local external forces. The measure of stress upon the balloon's wall at any particular point must involve the measurement of the differential between the balloon's internal pressure (constant) and the effective pressure (variant) immediately exterior of the balloon at the point of interest. Only the exterior applied pressure can vary in magnitude since the internal pressure is uniform.
In the large metallic structure, the externally applied force (potential) at any point in the adjacent earth region is the result of the structure's electrochemical equilibrium in association with the earth region environment. The earth region surrounding the structure is not homogeneous and therefore, the local external (potential) "forces" vary from point to point. These different externally applied "forces" cause the electrochemical energy exchange between the structure and earth region and produce corrosion by the evolution of hydrogen at areas of low potential and disassociation of metal atoms at areas of higher potential. Obviously, placement of a reference electrode in an area of low potential produces erroneous measurements for areas of high potential, i.e., the problem of prior art measurement schemes.
The structure is protected cathodically upon effectively equalizing the potentials (or forces) on the structure and the surrounding earth region. Cathodic current from a single anode at some distance from the structure probably will not equalize the entire structure. A multiplicity of anodes are required and each anode adjusted in current flow to equalize the potential "pressure" at all points on the structure. This latter arrangement provides a uniform cathodic reaction over the entire surface of the structure. Thus, cathodic protection requires the pressure alteration of the external environment surrounding the structure to provide equalization of electrochemical potential (substantially zero voltage differential).
It is not possible or practical to isolate the structure into a great number of individual parts without affecting the purposes for which it exists, even if all parts could be joined electronically so as to maintain a uniform structure potential. Also, it is not possible to ascertain the external "potential" force adjacent to each or any part of the structure when mechanically or integrally connected. The vast majority of "cathodic protection" practioners and electrochemists simply determine the in situ electrochemical "contact" and induced potentials of a structure by the measurement of the potential difference between the structure and the sancrosanct "reference" electrode(s) placed in the earth region. The existence of a "component" potential which is the result of the external force acting at a distance from the external surface (IR drop) is a substantial component of the measurable potential difference. As a result, this component potential causes serious errors when surveying cathodically protected systems, whether energized or not.
If each elemental part of the structure were disconnected electronically in turn and one by one from the remaining structure, the free part attains equilibrium with its environment, i.e., the rest of the structure. Subsequent measurements of the difference between the "internal" potential of the structure and that of the "electronically" isolated part would be a direct measure of the influence of the external force at a distance on the structure at that location, i.e., the "polarization" potential shift attributable to the anodic electrode as it affects the structure at that locale in the earth region. Upon reconnection of the elemental part to the structure, a measure of the current between that part and the structure could be made by zero resistance ammeter techniques. As a result, the current (density) and the potential shift to zero potential difference could be measured. Also, if the disconnected part is driven through a limited range of potential magnitude with respect to the remaining structure, both positive and negative several hundred millivolts about the open circuit value, the resulting potential/current responses can be utilized to determine (a) anodic reaction rate, (b) cathodic reaction rate, (c) unprotected corrosion rate, (d) protected corrosion rate, and (e) current density available from anodic ground bed.
The present invention provides a method for producing the above results with the same accuracy and facility as if a part of the structure could be electrically disconnected from the remainder of the structure without destroying its useful purposes.